Attempts have been made to improve the fuel economy of a vehicle by reducing the rolling resistance of a tire to suppress heat build-up. Tires with better fuel economy have recently become increasingly desired. Thus, several methods are used for better fuel economy, including reducing not only the heat build-up of a rubber for treads, but also reducing the heat build-up of a rubber for sidewalls or insulation, as well as reducing the thickness of a sidewall rubber.
Techniques used to reduce the heat build-up of a rubber include a technique of reducing the filling ratio of carbon black, and a technique of incorporating a filler such as silica to reduce the energy loss. These techniques reduce the rolling resistance of a tire, but are causing the problem of an increase in the electrical resistance of a tire because, for example, the amount of carbon black with good conductivity is reduced or the amount of silica with low conductivity is increased. An increase in the electrical resistance of a tire may lead to radio noise or cause an electrical discharge during fueling to ignite gasoline.
Meanwhile, a known method for suppressing an increase in the electrical resistance of a tire is to form a conductive path from the road surface to a rim using highly conductive rubber components, as taught in, for example, Patent Literature 1. Specifically, for example, as illustrated in FIG. 1, (1) a conducting rubber, (2) a breaker, (3) an insulation, an inner liner, a carcass and/or a sidewall, and (4) a clinch are formed of highly conductive rubber components; the conducting rubber is embedded in a tread so that it comes into contact with the road surface. With these, a conductive path is formed from the conducting rubber through the breaker, from the breaker through the insulation, inner liner, carcass and/or sidewall, and from these components to the clinch that is in contact with a rim. Thus, a conductive path can be formed from the road surface to the rim via (1) the conducting rubber, (2) the breaker, (3) the insulation, inner liner, carcass and/or sidewall, and (4) the clinch, so that the static electricity generated in the tire can be discharged. In this case, an undertread and a jointless band may also be formed of highly conductive rubber components and used to form a conductive path between the components (1) and (2).
As for the components (3) in the above conductive path, it would be enough if at least one of these components is formed of a highly conductive rubber component. Of the components (3), the sidewall highly contributes to a reduction in heat build-up leading to a reduction in the rolling resistance of a tire, and the insulation is not required in every tire; therefore, these components are considered to be unsuitable as components for securing conductivity. The present inventors have eventually concluded that it is necessary to ensure good conductivity for a carcass.
Meanwhile, a known technique for giving conductivity to a rubber composition is to add a conductive carbon black such as ketjenblack EC300J (Mitsubishi Chemical Corporation). Conductive carbon blacks, however, are a material generally used as, for example, a coating material, a colorant, a toner, or an electrode material for a cell and are unfortunately too expensive to be used for tires.
In consideration of this issue, Lion Corporation started marketing an inexpensive conductive carbon black under the name of Lionite. However, the present inventors have revealed in their studies that a rubber for carcasses (a fiber cord topping rubber) containing this conductive carbon black has good conductivity, but has reduced adhesion to fiber cords, thereby reducing the tire durability.